This invention relates to a computed tomography imaging method to reduce motion artifacts resulting from substantially periodic motion of the object being imaged. More specifically, the invention relates to an image artifacts suppression method for use with projected image techniques such as transmission computed tomography (CT) and emission computed tomography. As used herein, transmission imaging refers to imaging by detecting radiation transmitted through the body being imaged, and emission imaging refers to imaging by detecting radiation emitted from the body being imaged, e.g., such as that being emitted by radiopharmaceutical isotopes.
In one embodiment of a transmission computed tomography system, an x-ray source is collimated to form a fan beam directed along a slice plane through an imaged object to a series of x-ray detectors oriented within the slice plane. Each detector measures the intensity of transmitted radiation along a ray projected from the x-ray source to that particular detector element. The intensity is dependent on the scattering and absorption of the x-ray beam along that ray by the imaged object. The detectors are organized along an arc each to intercept x-rays from the x-ray source along a different ray of the fan beam and hence together to collect data for a complete projection. A given projection collected in this manner is identified by the fan beam's center most ray, termed the projection axis. The x-ray source and detectors are then rotated within the slice plane around the object being imaged so that the principal axis of the fan beam intercepts the object being imaged at a new angle The process is repeated to collect a number of projections each along a different such angle to form a tomographic projection set. The multiple projections of this set are then reconstructed through reconstruction algorithms known in the art to provide a tomographic view in the slice plane of the object being imaged.
Emission computed tomography may be performed in a similar manner. Briefly, a set of detectors are again rotated around the imaged object within a slice plane. The detectors receive radiation not from an external x-ray source, but rather from radioactive isotopes within the object itself. The radiation received by the detectors reveals scatter and absorption of the emitted radiation and also relative concentrations of the radioactive source within the object being imaged. The detector array receives a different projection as its position is moved to different angles with respect to the imaged object all within the slice plane. Each projection may be identified by the principal axis of the detector array, also termed the projection axis.
The acquisition of a computed tomographic image may take a significant amount of time. In both transmission and emission tomography, the need for multiple projections usually requires physical movement of the detector array and/or the x-ray source. Efforts at speeding the acquisition of projection data are limited by the speed with which the mechanical elements of the acquisition system, the detector and/or the x-ray source, may be relocated around the object to be imaged. Attempts to reduce scan times beyond certain limits can also adversely affect signal to noise ratio in the reconstructed image. Signal to noise ratio is directly related to the emitted or transmitted fluence during the acquisition of a given projection, and the fluence is effectively reduced at higher scan rates.
If the imaged object moves during the acquisition of the complete set of projections required for image reconstruction, the resulting image may show certain motion induced artifacts in the form of loss of resolution, streaking, "bubbling" in the area of dense objects, or doubling of moving objects termed "ghosts". Ghosts are more common when the motion is substantially periodic.
Most physiological motion is not perfectly periodic. The period may change over time and the moving object may not return to the same position even at identical points within each cycle. For the purposes of this discussion, the former criterion will be termed temporal periodicity and the latter criterion will be termed spatial periodicity.
Considerable effort has been invested in reducing the scan times required for a tomographic reconstruction. Practical scan times have been reduced from minutes to a few seconds over the last several years. Nevertheless, the periods of cardiac, peristaltic and respiratory motion are such that some motion is likely during data acquisition even under present shortened scan times. A number of methods are used to minimize such motion artifacts. Scanning protocols may be adopted to limit the motion: patients are instructed to lie still and to hold their breath during the scan, patient restraints and supports limit general muscular motion, and peristaltic motion may be reduced with drugs. These techniques are only partially successful and are often not feasible as in the case where the patient is traumatized or where scanning must be performed in an emergency situation.
As an alternative, gating techniques which acquire partial sets of projections timed with the periodic physiological activity, such as a breath or a heart beat, have been used with some success. In these techniques, the acquisitions of a full set of projections for an image is performed over a number of cycles of the physiological motion. Typically the projections are continuously acquired and then "sorted", according to the phase of the cycle during which they were taken to construct separate images for each phase of the periodic motion. With cardiac gating, for example, the EKG signal is used to coordinate the acquisition of data with the phase of the cardiac cycle, to obtain separate images at each phase of the beating heart after a number of cycles or beats of the heart.
Although the above described gating techniques allow for images with reduced motion artifacts, these techniques have some drawbacks. A given image images takes longer to obtain because data can be acquired only during a portion of each motion cycle. For reasonable acquisition periods, there will be missing projections from each projection set as a result of a failure of a particular projection axis to coincide with the proper motion phase during the acquisition time and less than ideal projection data will have to be used. Finally, because many cycles are required to obtain the entire projection set for an image, this technique is particularly susceptible to spatial non-periodicity in the physiological motion.